Place a polycarbonate (PCT) or polyester (PET) membrane under an electron microscope and you'll something similar to the picture here: a smooth surface perforated with neat, cylindrical holes. In this aspect, PCT and PET membranes stand out from other membrane types such as PTFE, nylon, or silver which provide irregular, tortuous paths for permeates to follow. So how are the regular little pores created? Are they drilled, punched, molded or torn into the membrane? If you guessed etched in with the help of a nuclear reactor, then you are absolutely right.
Every PCT or PET membrane filter starts as a roll of plastic film stock. The film is exposed in a controlled manner to charged particles in a nuclear reactor. The charged particles pass through the film, leaving behind sensitized tracks. The density of these tracks in the film depends on the amount of time that the film is exposed to the reactor. More time in the reactor with result in more tracks and greater pore density in the finished membrane.
The next step of the process involves treating the membrane with a caustic etchant such as NaOH. The etchant eats away at the polymer material starting at its weakest points: the tracks. The etchant widens the tracks into full-fledged pores, the size of which can be controlled by carefully monitoring the exposure time, concentration and temperature of the etchant.
Since all aspects of the membrane manufacturing process can be precisely controlled, a given batch of PCT or PET membranes can be manufactured to exacting specifications. And because the PCT and PET filters lack the tortuous paths used by other types of filters, they are ideal for applications where the retentate on the filter needs to observed. Sterlitech offers a wide range of polycarbonate membrane filters which can be found here as well as polyester filters which can be found here.
For more information on track etching, check out the following sources:
Apel, Pavel, and Reimar Spohr. "Introduction to Ion Track Etching in Polymers." Ion Track and Microtechnology Principles and Applications. N.p., n.d. Web. 29 July 2013. <http://ion-tracks.de/introduction1/index.html>.
Awasthi, Kamlendra. "Transport through track etched polymeric blend membrane." Bulletin of Material Science. 29.3 (2006): 261-264. Web. 29 Jul. 2013. <http://www.ias.ac.in/matersci/bmsjun2006/261.pdf>.
Sartowska, Bozena. "Nanopores with controlled profiles in track-etched membranes." Nukleonica. 57.4 (2012): 575-579. Web. 29 Jul. 2013. <http://www.nukleonika.pl/www/back/full/vol57_2012/v57n4p575f.pdf>
